|Monthly Tech-Tip |
Alternate Names: Red Art
Jan 2012: We are got reports of Redart properties possibly showing some variation, we arranged testing and on separate shipments arriving at different places and spanning a year. The results for drying shrinkage, water of plasticity, drying performance and fired shrinkage were very similar for all.
Redart is a red firing earthenware of moderate to low plasticity and low shrinkage. It fires light orange to dark red depending on firing temperature from cone 06 to 3. It fires much stronger and denser at cone 04 than 06 and achieves its best color/strength compromise at cone 02. It produces deep red color and stoneware properties at cone 1. It is airfloated to 200 mesh and has a long firing range. This is a very popular ingredient in clay bodies made across North America and it has been available for many years.
This material is quite high in iron thus the powder is a deep red color and quite messy to work with. Redart is also used by potters in glaze recipes, sometimes up to 60%.
Redart can be used as a primary component in brown and red burning modeling and throwing bodies (however significant plastic additions of ball clay (e.g. 20%) will be necessary to get sufficient plasticity). For plastic terra cotta bodies the highest proportion of Redart possible is desirable to achieve the needed color. For a body containing 75% Redart and no other plastics, at least 5% bentonite is needed to get enough plasticity for throwing.
Redart can be used to make casting bodies without any other additions. It will produce very good red color and will deflocculate and cast well. It is possible that a small addition of plastic clay might be needed to give it better strength to pull away from the mold. Also, additions of silica (for glaze fit) and a frit (for maturity) are also options.
While Redart can be used as a source of iron in high temperature bodies, it will produce brown rather than red coloration. This is because the fluxes are intimately mixed with the iron and fuse it to a darker color. To get red in oxidation or reduction at higher temperatures you must use refractory clays with iron or iron bearing fireclays. However there is good reason not to use low fire reds as fluxes in high fire bodies: The fluxes in the red clays don't dissolve cristobalite like feldspar does, the result can be thermal expansion related ware failure.
This material is mined from a large deposit and the company is confident in its consistency and long term availability.
Sample body recipes for low fire terra cotta:
Redart Casting mix #1062
25 Nepheline Syenite
15 Large Particle size kaolin
0.15 Soda Ash
0.65 N Brand Sodium Silicate
For redder color you can replace the kaolin with more Redart but dry strength will be lower. You can even use straight redart for casting.
Redart very plastic throwing body #3322A
40 Banta Red Clay
10 Ball clay
This body has remarkably good plasticity yet its drying performance is also very good. It is melting by cone 4 and is quite vitreous by cone 02. Talc in low fire bodies can really improve drying while not reducing plasticity as much as other fillers, talc also increases the thermal expansion for better fit to commercial glazes.
The Redart clay bars (left) are fired at cones 06, 04, 2, 4 & 5 (top to bottom). The Plainsman Blue Grey Plastic bars (right) are fired at 06, 04, 03, 02, 2 & 4. The SHAB test procedure (used to make these) gives us the firing shrinkage and porosity at each temperature, these are direct indicators of the fired maturity. Notice how much the fired color changes with increasing temperature. The fired maturity is pretty similar but the BGP is a little browner in color. It is also much more plastic (the drying shrinkage quite a bit higher).
Plainsman M2 (left) vs. Redart (right). These bars are fired cone 04, 02, 2, 3, 4 (top to bottom). Fired color is almost identical. M2 has a little more soluble salts (however M2 has no chemical additions to precipitate them, it is ground as the pure mined material). M2 is more plastic (although still not as plastic as a typical pottery clay). However, Redart will make a good casting slip while M2 does not respond to deflocculants.
These bars have been fired at cones 4, 2, 02, 04 (top to bottom) using the SHAB testing procedure. We can measure fired shrinkage and porosity in each to get an indication of their fired maturity. The Redart (left) is much more vitreous and reaches almost zero porosity by cone 4 whereas the Lizella still has 11% porosity at cone 4. Lizella also has a much higher drying shrinkage (because it is way more plastic).
A cone 10R sculpture clay containing 40% ball clay, 10% kaolin, 10% low fire redart (for color and maturity), some quartz and 25% 20x48 grog. This fine grained base produces a body that feels smoother than it really is and is very plastic. It is even throwable on the wheel.
Stoneware at cone 02? Yes. These test bars are fired to cone 02. The top body is 50:50 Redart and a silty raw material from Plainsman Clays (named 3D) plus some bentonite and 1% iron. The bottom one also has 5% Ferro Frit 3110. The porosity: The bottom one is 3%, the top one 8%. So each 1% frit reduces the porosity by 1% in this case.
This is a grog clay with 25% Christy Minerals STKO22S grog (20 mesh one size). This piece is about 8 inches tall fired at cone 10R. This body is a Redart, Ball clay base that totally vitrifies to a chocolate brown. But with the added refractory grog it is fairly stable in the kiln and is much more vitreous than other grog bodies. Because it is such a plastic smooth base and because the grog is only one size, this is actually throwable. And it is very resistant to splitting during hand building.
These bowls are fired at cone 03. They are made from 80 Redart, 20 Ball clay. The glazes are (left to right) G1916J (Frit 3195 85, EPK 15), G191Q (Frit 3195 65, Frit 3110 20, EPK 15) and G1916T (Frit 3195 65, Frit 3249 20, EPK 15). The latter is the most transparent and brilliant, even though that frit has high MgO. The center one has a higher expansion (because of the Frit 3110) and the right one a lower expansion (because of the Frit 3249). Yet all of them survived a 300F to icewater IWCT test without crazing. This is a testament to the utility of Redart at low temperatures. A white body done at the same time crazed the left two.
The soluble salts dissolved in the water of plasticity of this red body have migrated through the white engobe during drying of these earthenware cups. The cups were upside down so all the solubles have been left on the outside surface. The red body is made using a high percentage of Redart clay (a widely available commercial low-fire low-plastic clay in North America). It is plasticized using added ball clay. The brownish material is organic, because after bisque firing it has disappeared.
This slurry is 100% Redart.
The two clay bars were fired side-by-side at cone 01. The back bar is of a raw clay dug from a creek bed in Alberta, Canada. Notice how it puffs up inside and eventually splits open the outer layer (which has sealed in the gases of decomposition). The front bar is that same clay, but mixed 50:50 mix with Redart. It is stable and strong as a stoneware. You can see all the lab tests I did on this in my insight-live account at http://goo.gl/KiUoU0
Fired to cone 13 in a Manabigama wood fired kiln.
These three melt flows and mugs were fired at cone 6 (using the C6DHSC firing schedule). The benchmark recipe is 80% clay and 20% Ferro Frit 3195. The center melt flow and mug (made from a Plainsman 3D-based stoneware) employs original Albany Slip as the clay portion. The one on the far left uses an Albany Slip substitute that was developed by calculating a mix of RedArt and other materials to have the same chemistry as Albany Slip. The one on the right employs Alberta Slip. Notice that, although the Alberta Slip version has a very similar melt flow, on the mug it is apparent that it needs a little iron oxide for a better match (e.g. 1-2%). And the glaze on the left: The chemistry of RedArt is different enough from Albany that some compromises in chemistry-matching were needed to avoid over-supplying the iron even more (and firing even darker than this). Although this Redart version runs in a very similar pattern on the melt flow, the character of the glaze is somewhat different on the mug (a better match can be achieved by increasing the frit percentage slightly, or firing to cone 7).
Understanding the Deflocculation Process in Slip Casting
Understanding the magic of deflocculation and how to measure specific gravity and viscosity, and how to interpret the results of these tests to adjust the slip, these are the key to controlling a casting process.
Understanding the Terra Cotta Slip Casting Recipes In North America
This article helps you understand a good recipe for a red casting body so that you will have control and adjustability.
|Materials||Newman Red Clay|
|Materials||Banta Red Clay|
Resco Clays Data Sheet
Clays that are not kaolins, ball clays or bentonites. For example, stoneware clays are mixtures of all of the above plus quartz, feldspar, mica and other minerals. There are also many clays that have high plasticity like bentonite but are much different mineralogically.
|Firing Shrinkage||Cone 04: 3% Cone 2: 8%|
|Water absorption||Cone 04: 11% Cone 2: 1%|
|Body Maturity||Red terra cotta clays are typically 6-8 cones more vitreous than is possible with feldspar-white clay mixes. Thus where a red clay can be tolerated, materials like this can be employed to create cone 02-2 stoneware.|